1. Field of the invention
The invention concerns contact lenses. It is more particularly concerned with lenses that are not circumferentially symmetrical, such as lenses for correcting astigmatism. To be more precise, it concerns a method of marking angular tolerances to facilitate putting in such lenses.
2. Description of the prior art
Contact lenses for correcting astigmatism have the particular feature that at least one of their faces is not circumferentially symmetrical about the optical axis. Consequently, they must be placed on the eye of the wearer in the correct angular position, failing which the correction of vision is imperfect.
Conventional lenses for correcting astigmatism therefore have symbols in their peripheral part, which has no optical function, enabling the clinician to verify correct orientation of the lens on the eye. These symbols include angular markings for assessing the positioning error and where applicable comparing it to a tolerance with respect to the optimal position of the lens, assuming that this tolerance is known.
The invention is directed to a novel method of producing angular tolerance markings specific to each lens and allowing for the fact that the acceptable angular tolerance depends on the cylinder of the lens. In addition to correcting astigmatism, the lens can incorporate any other type of correction. One objective of the invention is to determine directly whether the angular position of the lens is within the tolerable range or not. Another objective of the invention is to save clinicians time and facilitate their work. The invention can be used on any type of lens for correcting astigmatism, the lens itself constituting another aspect of the invention.